Emergy

Emergy is a type of energy (exergy) that is consumed in direct and indirect transformations needed to make a product or service. Emergy is a measure of quality differences between different forms of energy. Emergy is an expression of all the energy used in the work processes that generate a product or service in units of one type of energy. Emergy is measured in units of emjoules, a unit referring to the available energy consumed in transformations. Emergy accounts for different forms of energy and resources (e.g. sunlight, water, fossil fuels, minerals, etc.) Each form is generated by transformation processes in nature and each has a different ability to support work in natural and in human systems. The recognition of these quality differences is a key concept.

The theoretical and conceptual basis for the emergy methodology is grounded in thermodynamics, general system theory and systems ecology. Evolution of the theory by Howard T. Odum over the first thirty years is reviewed in Environmental Accounting and in the volume edited by C.A.S. Hall titled Maximum Power.

Beginning in the 1950s, Odum analyzed energy flow in ecosystems (e.g. Silver Springs, Florida;Enewetak atoll in the south Pacific;Galveston Bay, Texas and Puerto Rican rainforests, amongst others) where energies in various forms at various scales were observed. His analysis of energy flow in ecosystems, and the differences in the potential energy of sunlight, fresh water currents, wind and ocean currents led him to make the suggestion that when two or more different energy sources drive a system, they cannot be added without first converting them to a common measure that accounts for their differences in energy quality. This led him to introduce the concept of "energy of one kind" as a common denominator with the name "energy cost". He then expanded the analysis to model food production in the 1960s, and in the 1970s to fossil fuels.

Odum's first formal statement of what would later be termed emergy was in 1973:

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